Fields missing in sender FCC- fixed length

The system is PI 7.4 dual stack, and the file adapter version is 7.00 or 7.40.

We are converting incoming flat file(fixed length) with FCC in sender channel, and strange issue occurs when last fields missing in the flat file.

Flat file (the second line has last 15 digits missed, and the third line is just blank with no space):

FCC configuration:

There was no missingLastFields (=add) or lastFieldsOptional (=YES ) specified, so the conversion should fail in adapter engine, and in fact, it did fail one time in production, error message is exactly about missing last fields etc..

The solution should be straightforward - just add one of parameters above. However, during the issue replication in development environment, this configuration is working properly without any issue - the last field(QUANTITY) is generated with value empty in the XML..

I tried both adapter version 7.0 and 7.4, even specified lastFieldsOptional = NO explicitly (actually lastFieldsOptional is obsolete, and test shows it doesn't have any impact with YES or NO, never use it anymore... correct me if wrong), the QUANTITY is always get generated with empty value.

So the only reason I am supposing is the default value of "missingLastFields" is ADD having "fieldFixedLength" specified, which overwrites the function lastFieldsOptional (=NO), but I didn't find official document to prove this..

Do you have any input for this strange issue(fail in pro, but work in dev)? and is my assumption correct(default value of "missingLastFields" is ADD)?

PS..I understand it is recommended that always have "missingLastFields" and "additionalLastFields" specified to ensure a well-defined runtime behavior, but now we are having a couple of such interfaces without these parameters in production.. and it is not that easy to make so many changes in pro...